Applicants' invention relates generally to radiocommunication systems, e.g., cellular or satellite systems and, more particularly, to techniques for supporting and enhancing emergency calling procedures in such systems.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that the radiocommunication system be able to independently determine a location of the mobile unit, particularly in the case where the subscriber does not know his or her precise location. Moreover, it is expected that the FCC will soon require that network operators forward the position of an emergency caller to the emergency service provider.
There are many techniques available to generate mobile unit location information. In a first category, the mobile unit could estimate its own position and send a message with its coordinates when placing an emergency call. This could be accomplished by, for example, providing the mobile unit with a Global Positioning System (GPS) receiver that receives location information from the GPS satellite network. The mobile unit can then transmit this information to the system, which would then forward it to the emergency service provider. This requires, however, significant modification of existing mobile units to include GPS receivers, as well as additional signalling between the mobile units and base stations.
Alternatively, the base stations which transmit signals to, and receive signals from, the mobile units could be used to determine the mobile unit's location. Various techniques, including attenuation of a mobile unit's signal, angle-of-arrival, and difference between the time-of-arrival (TDOA) of a mobile unit's signal at different base stations, have been suggested for usage in providing mobile unit location information. See, for example, the article entitled "Time Difference of Arrival Technology for Locating Narrowband Cellular Signals" by Louis A. Stilp, SPIE Vol. 2602, pp. 134-144. These solutions also have their drawbacks including the need to modify the many existing base stations, e.g., to provide array antennas to support angle-of-arrival techniques or to synchronize base station transmissions to support TDOA techniques.
A third category of strategies for locating mobile units in radiocommunication systems involves the provision of an adjunct system, i.e., a system which may be completely independent of the radiocommunication system or which may share various components (e.g., an antenna) with the radiocommunication system but which processes signals separately therefrom. This may be advantageous, for example, as an expedient solution to providing mobile unit location without modifying the large number of existing base stations in a system. For example, consider the equipment illustrated in FIG. 1 wherein the adjunct scanning units are not co-located with the base stations of radiocommunication system. Therein, a base station 1 supports radiocommunication within cell 2 and, in particular with mobile unit 3. An adjunct system, partially shown by way of scanning units 4, 5 and 6, monitors accesses to the system by mobile unit 3. When mobile unit 3 makes an emergency access, adjunct units 4, 5 and 6 use the mobile unit's transmissions on either a control channel or a traffic channel to provide information to a location processing center 7. The location processing center then uses the information provided by the various adjunct units to, for example, triangulate the position of mobile unit 3 and report this position to an emergency service center 8. More details regarding exemplary usages of adjunct systems can be found in U.S. Pat. No. 5,327,144 to Stilp et al., entitled "Cellular Telephone Location System", the disclosure of which is incorporated here by reference.
However, using an adjunct system to locate mobile stations in support of, for example, emergency call processing, also poses several challenges. One of these challenges involves monitoring the location of mobile units operating in hierarchical, or multi-layered, cell structures used in today's radiocommunication systems in high traffic areas to provide adequate capacity and efficient bandwidth utilization. These hierarchical cell structures include, for example, overlapping combinations of macrocells, indoor microcells, outdoor microcells, public microcells and restricted or private microcells.
FIG. 2 illustrates an exemplary hierarchical cell structure. An umbrella or macrocell 10, represented by a hexagonal shape in this figure, provides a first, relatively large area of radiocommunication service. Within umbrella cell 10 are microcell 20 (represented by the area enclosed within the dotted line) and microcell 30 (represented by the area enclosed within the dashed line) which provide radiocommunication service to corresponding areas along city streets. Even smaller cells 40, 50 and 60, sometimes referred to as picocells, may be established on floors of a building to provide radiocommunication service to high density, indoor users.
Each of the cells 10-60 illustrated in FIG. 2 will include a base station (not shown) which support radiocommunication within their illustrated cell areas. In some cells, particularly those like indoor picocells 40, 50 and 60, radiocommunication may be conducted at relatively low power levels due to the proximity between the remote units and the base station within each picocell. Consider a remote unit, e.g., a portable or mobile phone, which is travelling down the city streets and up into the building including picocells 40, 50 and 60. During various portions of this transit, potentially all of the base stations associated with the umbrella cell 10, the microcells 20 and 30 and the picocells 40, 50 and 60 may provide service to the remote unit. Thus, should an emergency occur at some point during the remote unit's transit and the user place an emergency call, any one of cell 10-60's base stations (and corresponding control channels) might conventionally be used to request emergency services.
This situation provides a dilemma for systems wherein, for example, mobile unit location information is determined using an adjunct system. More specifically, it may not be possible to scan the transmissions of mobile and base stations anywhere within umbrella cell 10 using only one (or even more) adjunct scanning units. For example, if the transmissions within picocells 40, 50 and 60 are specified to be made at a relatively low power, then an adjunct scanning station 70 may not receive an emergency call transmitted by a mobile unit served by one of these cells and, therefore, would not be able to provide location information to an emergency center. One solution to this problem would be to provide adjunct units within each of these cells, as well as possibly elsewhere within umbrella cell 10, to provide sufficient adjunct coverage to locate a mobile station placing an emergency call through any of the base stations in the cell.
Increasing the number of adjunct units is, however, economically unattractive due to the increase in cost associated with implementing such a system. Accordingly, it would be desirable to provide some alternative techniques for handling emergency call processing and location of mobile stations which does not require the provision of a large number of adjunct units.